The need to automatically fill containers with tightly regulated amounts of a bulk material or discrete items is well known. Applications include putting pills into bottles, seeds into bags, and pyrotechnic material into air bag components. In some cases, such as supplying the fasteners for knock down furniture, the primary motivation for accuracy is economic. Shortage are expensive to rectify, overages are wasteful. In other cases, such as pharmaceuticals and pyrotechnics, the motivation is safety. Whatever the motivation, the need for accurately and quickly, dispensing materials is increasing. As with all production operations, higher speed translates to reduced costs, but the accuracy can not be sacrificed.
Many different approaches to accurately and quickly dispensing bulk materials have been used. Among the most common are volumetric cups or shuttles; augers, vibratory bowl feeders and linear vibratory feeders. Of these, linear vibratory feeders are the most versatile. They can handle a broad range of materials including abrasives and materials which tend to compact
While linear feeders can be designed to handle a wide range of materials and flow rates, a particular design is limited to a relatively small range of rate or accuracy. This is due to the inherent limitations of the design. A linear vibratory feeder operates by vibrating a linear feed trough to move the material from a supply (usually a cup or hopper) to the discharge end of the trough. At the end of the trough, the material falls off the end into a receiving container. The discharge rate is directly proportional to the width of the end of the trough and the bed depth of the material at the end. Wider troughs and deeper bed depths provide proportionally higher discharge rates. Unfortunately, the accuracy of a feeder is inversely proportional to exactly these same characteristics. An accurate feeder will have a very narrow end and shallow bed depth. A feeder's accuracy is roughly proportional to the quantity of material which will be discharged the end on one cycle of the motor. Where discrete items are being dispensed, such as pills or pellets, the highest accuracy is obtained where only a single pill or pellet is available at the end of the trough. These competing demands have led to the use of two separate troughs: one for bulk feed; and one for trickle, or top-off, feed. One way of doing this involves separating a single trough into two separate paths, one with a small capacity and small discharge end. The two paths are physically attached and driven by a single vibratory motor. A gate blocks the flow from the larger, bulk, path when the target amount is approached and the trickle path finishes off the fill. A problem with this approach is that the material in the bulk path continues to feed while the trickle path is in use, and the material builds up behind the gate. This may result in compaction of the material, block the feeder, or may result in a surge of material being dispensed once the gate is reopened. In a worst case, the surge may be greater that the entire amount desired to be dispensed, resulting in an overfill.
Some systems utilize two or more feeders which supply a different rate or quantity of material. The receiving container is then moved between stations to achieve a complete fill. This increases the complexity of the system and can increase the cycle time required for a fill.
Some multiple feeder systems become quite large due to the size of the individual feeders. This is especially true where multiple bowl feeders are used. Because of their size, if multiple feeders are ganged together to provide a system with multiple fill stations, the resulting system is quite large.
Bowl feeders and some of the linear feeder designs use a sweep or other method to reduce the bed depth on the trickle path by pushing excess material over the edge of the feeder, typically at a single point In some designs it falls into the attached bulk path and in others it falls into an overflow catch and needs to be recycled into the supply hopper. Such recycling designs increase the cost of the system through an additional material path or manual transfer of the overflow material to the supply hopper. Further, recycling of material can lead to degradation (especially breaking up of pellets) or segregation of particulate sizes.
Other problems with bulk material feeders include bridging and compaction. Either can result in a blockage of the feeder or of an uneven flow of the material as the problem occurs and then breaks up. This is especially troublesome where a gate is positioned near the bottom of the supply hopper or reservoir.
There is a need for a feeder system which provides two or more feeders which discharge to the same location so that the receiving container need not be moved. The feeders should be independently controllable and independently driven so that the action of one does not affect the other. The feeders should handle a variety of bulk materials with minimal or no risk of compaction or bridging. To this end, no control gates should be used in the feeders. The design of the feeders should be compact to allow for ganging of multiple sets of feeders or for a compact layout for a single pair system. Ideally the feeder system will provide a supply path which provide a steady flow of material to the feeders at a relatively constant rate, also without compaction or bridging. The delivery rate of the supply path should be adjustable independently of the feeder rate.